Many communications networks today provide high bit-rate transport over a shared medium, such as a Passive Optical Network (xe2x80x9cPONxe2x80x9d), a Cable television (xe2x80x9cCATVxe2x80x9d) coaxial or hybrid fiber/coax network, or a wireless network. These shared medium networks typically use time, frequency or code division multiplexing to transport data signals from a central terminal to several remote customer terminals and Time Division Multiple Access (xe2x80x9cTDMAxe2x80x9d) to transport data signals from the remote terminals to the central terminal. TDMA is characterized by non-continuous or burst mode data transmission.
In existing optical networks, especially in a PON system, each xe2x80x9cpacketxe2x80x9d of information from a remote terminal is multiplexed in a time sequence on one fiber and transmitted in a burst-like manner. Such burst mode packets transmitted from one or more remote terminals to a receiver at the central terminal present a number of constraints to the receiver. The receiver is likely to receive closely spaced packets that have a wide range of optical power levels. A weak signal in a received packet needs to be amplified sufficiently so that it can be used, but if a strong signal in a packet is amplified too much the signal can be lost due to blinding (saturating) the amplifier. The difference in power levels of the incoming packets may be such that the noise induced on a powerful signal in one packet may be greater than a weak signal in a subsequent packet.
In burst mode, a guard time is provided between packets. Typically, the guard time is the time to transmit 8 bits. For example, at 6 nano-seconds (xe2x80x9cnsxe2x80x9d) per bit, the time to transmit 8 bits is 48 ns. Thus, the receiver must be able to respond to a packet with a different power level in less than 8 bit times. The power ratio between sequential packets can be greater than 1000:1. The receiver also has to respond to a weak signal in a subsequent packet within the guard time between packets.
An incoming packet can include a long string of bits set to logic xe2x80x98onexe2x80x99 or logic xe2x80x98zeroxe2x80x99. The receiver has to maintain the output at a constant logic level while the string of bits set to logic xe2x80x98onexe2x80x99 or logic xe2x80x98zeroxe2x80x99 is being received. However, the string of bits set to logic xe2x80x98onexe2x80x99 or logic xe2x80x98zeroxe2x80x99 may be longer, up to three times longer or more, than the guard time between packets.
A burst mode receiver can be AC coupled or DC coupled. Prior art AC coupled receivers cannot maintain the voltage level for a long string of xe2x80x98onesxe2x80x99 or xe2x80x98zerosxe2x80x99 in a packet and also respond to a weak signal in a subsequent packet within the guard time between packets. Therefore, the AC coupled receiver may drop bits in the next packet while adapting to the new power level.
In a DC coupled receiver, a rapidly responding Automatic Gain Control (xe2x80x9cAGCxe2x80x9d) is used to keep an amplifier from saturating on the noise or logic xe2x80x98zeroxe2x80x99 power level on a strong incoming signal. However, AGC can produce severe Pulse Width Distortion (xe2x80x9cPWDxe2x80x9d) after receiving a long string of xe2x80x98onesxe2x80x99 or xe2x80x98zerosxe2x80x99 in a packet at a high power level. Furthermore, the AGC must be extremely fast. This limits the incoming data rate at which the DC coupled receiver can operate because as the incoming data rate increases, so does the speed required for the AGC.
In accordance with the present invention, an AC-coupled pre-amplifier outputs an amplified signal with an undriven time constant that is longer than a driven time constant.
The AC-coupled pre-amplifier has a capacitor for AC-coupling a signal and an amplifier. The amplifier has an inverting input and a non-inverting input. The non-inverting input is coupled to the capacitor to receive the AC coupled signal. An input resistor is coupled between the inverting input and the non-inverting input. A feedback resistor is coupled between a positive output of the amplifier and the non-inverting input of the amplifier. The ratio of the input resistor the feedback resistor and open loop gain of the amplifier is selected to produce an undriven time constant for each undriven edge of the AC coupled signal. The undriven time constant is proportional to the capacitor and the input resistor. The undriven time constant is longer than the driven edge time constant but shorter than an inter-packet guard time.
The edge rate of the signal, and the ratio between an input drive current driving the signal and the capacitor control the driven time constant for each driven edge of the AC coupled signal.
A second feedback resistor may be coupled between a negative output of the amplifier and the inverting input of the amplifier and a second capacitor included for AC-coupling a second signal. The inverting input of the amplifier is connected to the second capacitor to receive the second AC-coupled signal. The ratio of the input resistor the second feedback resistor and the open loop gain of the amplifier is selected to produce an undriven time constant for each undriven edge of the second AC coupled signal. The ratio is chosen such that the undriven edge time constant is longer than the driven edge time constant but shorter than an inter-packet guard time.
The edge rate of the second signal, and the ratio between the input drive current driving the second signal and the second capacitor control the driven time constant for each driven edge of the second AC coupled signal.
A method for pre-amplifying a signal, AC-couples the signal through an AC-coupling capacitor to output an AC-coupled signal. The AC-coupled signal is amplified in an amplifier. The amplifier has a positive feedback resistor coupled between a non-inverting input and a positive output and an input resistor coupled between the non-inverting input and an inverting input. The ratio between the feedback resistor, the input resistor and the gain of the amplifier are selected to output an amplified AC-coupled signal having a driven edge time constant corresponding to each driven edge of the incoming AC-coupled signal and an undriven edge time constant extended and longer than the driven edge time constant corresponding to each undriven edge of the AC-coupled signal. The undriven edge time constant is less than a guard time between burst mode signals.